1. Field of the Invention
The present invention relates to a tilt controlling method and apparatus in a high density optical record medium system
2. Description of the Related Art
In general, an optical record medium is generally classified according to capability of rewriting into a Read Only Memory (ROM), a WORM which can be recorded once and a rewritable medium which can be repeatedly recorded.
Here, examples of the ROM type optical record medium include a Compact Disk Read Only Memory (CD-ROM), a digital Versatile Disk Read Only Memory (DVD-ROM) and so on. Examples of the WORM type optical record medium include a Recordable Compact Disk (CD-R), Recordable Digital Versatile Disk (DVD-R) and so on. Also, examples of the rewritable optical record medium include a Rewritable Compact Disk (CD-RW), a Rewritable Digital Versatile Disk (DVD-RW, DVD-RAM) and so on.
The foregoing rewritable optical record medium, for example the optical disk, has signal tracks constituted by lands and grooves so that tracking can be controlled even in a blank disk in which information signals are not recorded. Lately, the information signals are recorded in the tracks of the lands and the grooves to raise record density.
Also, a laser beam of an optical pick-up for record/reproduction has a single wavelength, and the opening ratio is enlarged to reduce the size of a light beam for record/reproduction.
Further, the high-density optical disk reduces a distance between signal tracks or a signal track pitch to raise the record density.
In this case, such an optical disk can be distorted during injection and hardening processes of a resin in a manufacture process, and accordingly have the eccentricity and the disk inclination even with a central hole. Further, the tracks of the disk generates the eccentricity caused by declination of the central hole even if they are correctly recorded in a spiral shape with a predetermined pitch. Therefore, the disk rotates with the eccentricity so that the central axis of a motor may hardly align with the center of the tracks.
Therefore, since it is difficult to correctly read a signal of only a desired track, the CD and DVD have standards determined about the variation and executes a tracking servo so that the light beam can always trace the desired track even in the eccentricity.
In other words, the tracking servo generates an electric signal corresponding to a beam trace condition and moves an object lens and an optical pickup body in the radial direction based upon the electric signal to correct the position of the beam thereby enabling correct trace of the track.
Meanwhile, the beam deviates from the corresponding track not only in the foregoing eccentricity of the disk but also in the inclination of the disk, which can take place due to a mechanical problem such as an error in mounting the disk to a spindle motor. In other words, focusing and tracking are deflected without having correct vertical alignment. Such a condition that the disk is inclined is called the tilt.
Such tilt has not been considered as a serious problem in the CD having a large tilt margin due to a wide track pitch. Here, the tilt margin means the magnitude in which compensation can be made even if the disk is inclined at a certain degree.
However, the DVD has a narrow track pitch due to high density and thus a small tilt margin so that the beam influences the next track even if the tilt takes place in a small amount or the disk is slightly inclined. In this case, the tracking servo is not sufficient to compensate the tilt.
In other words, when the light beam crosses over the adjacent track, it will be judged that the tracking servo incorrectly traces the track even if the focus of the object lens is at the center of the tracks.
In this case, data cannot be read correctly in reproduction or correctly written in the corresponding track in record so that dual distortion takes place in reproduction of the data written in this fashion.
Therefore, as a method to solve such a tilt problem, a dedicated tilt sensor or a tilt-dedicated light receiving device is additionally provided to detect the tilt of the disk.
However, such a method has a problem that efficiency is lowered and the size of a set increases.
A rewritable data zone of the DVD-RAM has head fields, and each of the head fields has a Variable Frequency Oscillator (VFO) area for generating a reference clock for adjusting a bit synchronization of a lead channel, in which the size of a signal or a tracking error signal is detected from the VFO area to control the tilt. However, the tilt cannot be detected if each sector of the data zone does not have a head.